Lecture 1 Parasites Evolutionary parasitology Flashcards
What were control measures set in place of yellow fever?
New control measures:
- fumigation
- habitat removal
- quantine
What are 5 things can we learn from yellow fever?
- Diseases can have complex life cycles
- Not all hosts & parasites are the same
- Complex physiological & molecular mechanisms involved
- Parasites & hosts are populations
- Diseases can be controlled if understood
Why do diseases have complex life cycles?
- difficult to i.d. disease agent
- difficult to control
- important for parasite ecology
- > disease spread, persistence
- important for evolution of host
- > parasite interactions
e. g. disease avoidance
- > parasite interactions
Why are not all hosts & parasites are the same? Give reference to Yellow fever.
- West Africans more resistant than French/British soldiers
• but not all sick, not all die
• within and among population VARIATION - YFV strains
Why is knowledge of complex physiological & molecular mechanisms involved important?
- very important for host-pathogen coevolution
- > dictate virulence & resistance
- critical for disease control
Describe the idea that parasites & hosts are populations.
•small & large scale interactions (growth, persistence)
(How many resistant individuals
are enough?)
What are things that need to be understood in order to try to control diseases?
• life cycle, transmission, within-population variation
- vector control
-reduce vector exposure
in addition to either one of these types:
Type A:
• anti-malarial drugs
-resistance
Type B:
vaccine
-“herd immunity
Do we really need to worry about infectious diseases?”
Some emerging (or re-emerging) diseases of note:: -avian and swine flu malaria • parasite adaptation & evolution • changing environment • changing hosts
What is evolutionary parasitology?
Partners in close relationships exert pressure
• “regular” selection pressure + coevolution
What is the Red Queen hypothesis?
-same rules & mechanisms apply (“regular” selection pressure + coevolution) • host & parasite genotypes & phenotypes interact -evolve together • only phenotypes interact ->dictated by genotype • some host genotypes more successful (e.g. resistance) • some parasite genotypes more successful (e.g. infectivity) • more successful genotypes selected for & passed on
How can Darwin’s natural selection criteria apply?
- Individuals in population must vary in phenotype
- Offspring must compete to survive & reproduce
- Some offspring will have advantage over others
- Heritable portion of phenotypic variation passed on
Describe Darwin’s selection criteria ‘individuals in population must vary in phenotype’ and what it relates to?
• extremely common
- > host susceptibility to infection
- > pathogen response to host defences
Describe one of Darwin’s selection criteria. Why must offspring compete to survive & reproduce?
• not enough resources for all
ex. 300 eggs/day, so many eggs competing for resources
Describe one of Darwin’s selection criteria. Why should some offspring have advantage over others?
better survival and/or
reproduction
->selection for that genotype
Describe one of Darwin’s selection criteria. Heritable portion of phenotypic variation passed on.
• favourable genotypes increase in frequency in population ->favourable phenotypes increase in frequency ->parasite can adapt to host (vice versa)
What are some caveats to Darwin’s selection criteria?
• hosts & parasites do not “choose”!
• selection only on
existing variation
-> no resistant phenotype, no adaptation
• selection only sees phenotypes
-> evolution only if genetic basis to phenotypic variation
- Bad genes or bad condition?
(e. g.) no evolution if resistance depends only on host nutrition (not heritable)
What are Niko TInbergen’s 4 basic questions for any trait?
1. Mechanism underlying trait? • typically physiology & molecular biology 2. Evolutionary purpose of trait? • carrier’s survival & reproduction 3. Trait ontogeny? • how trait is formed during course of individual’s development 4. Evolution of trait in phylogeny? • historic course of evolution
What are units of selection (individual vs. population, inclusive fitness)
-all selection, occurs at the individual level!
-counterintuitive: parasites should evolve to cause little harm to host
• loss of hosts = loss of parasites
-Increase in each parasite’s inclusive fitness the most potent evolutionary force!
What happens when parasites are related?
selection for decreased virulence
Why care about natural selection & parasites?
-critical aspects (virulence, resistance, transmission)
• most parasite traits adaptations to this lifestyle
->life history
Describe life history importance in caring about natural selection & parasites?
-lifetime fitness of individual
• mean population fitness
• trade-offs (e.g. reproductive maturity vs. lifespan)
Which strategies are “adaptive?”(individual & optimality, population & ESS)
Want maximum possible fitness for parasite/host
1. Optimality
• phenotype has highest possible fitness in given environment
• individual trade-offs:
-cannot “decide” on different transmission mode
–> evolved trait (population level)
• can control individual rate of development in host
–> trade-off: multiplication rate vs. infection duration
- Evolutionarily stable strategies (ESS)
• phenotype has highest possible fitness in given population
• ESS cannot be bested once becomes common
How to study evolution of hosts & parasites?
-observation
-experimentation (susceptible strain)
-theory (models, optimality, ESS analysis)
• comparative studies
–>“nature’s experiments”
—>host speciation drives parasite speciation
Describe the ‘optimal’ adaptive trait strategy optimal strategy and why its in the individual level?
- Optimality
• phenotype has highest possible fitness in given environment
• operates within strict individual constraints
• individual trade-offs still possible
-cannot “decide” on different transmission mode
–> evolved trait (population level)
• can control individual rate of development in host
–> trade-off: multiplication rate vs. infection duration
Describe the ‘ESS’ adaptive trait strategy optimal strategy and why its in the population level?
Evolutionarily stable strategies (ESS)
• phenotype has highest possible fitness in given population
• ESS cannot be bested once becomes common
—> how could “rock” invade a population of “paper”?
ex. Rock-paper game, paper is clearly ESS (b/c paper beats rock) and can invade a population of all Rock strategists when rare.
